Worrals. Like most documents regarding this subject that are available to the public it is pretty much full of motherhood statements like this one "The scale of threat cannot be estimated with certainty beyond the next few years" which states the bleeding obvious about the future. I have trouble believing the first part of the sentence which implies that "they" can be certain of "scale of the threat" in the short term.
Terrorism is like drug smuggling. You only know what you see. You have "known unknowns" such as "someone is planning a terror op somewhere" or "we've intercepted x kg of drugs, but, no, we don't know how much has got past us".
Of course, the problem for us looking in (to the intelligence community) from outside is that we don't know what they know. We have to assume the know what they are doing and are making the correct guesses. One can only hope that they are better at it now than when the "WMD intelligence" started the war in Iraq!
Minimbah